Electrochemical cell without an electrolyte-impermeable barrier

ABSTRACT

In one aspect there is provided an electrochemical cell without an electrolyte-impermeable barrier. In another aspect there is provided an electrochemical cell comprising a liquid electrolyte, a cathode and at least one cathode product able to be produced at the cathode, and an anode and at least one anode product able to be produced at the anode. The at least one anode product and the at least one cathode product are substantially separated, and the cell is without an electrolyte-impermeable barrier positioned between the cathode and the anode. There is a relatively low ratio of electrolyte volume to electrode geometric surface area of the cathode or the anode (electrolyte volume (m 3 )/electrode surface area (m 2 )). The cell can be operated at a relatively low current density. Optionally, an electrolyte-permeable separator may be employed.

TECHNICAL HELD

The present invention relates to electrochemical cells. In one form, thepresent invention more specifically relates to the elimination of theneed for an electrolyte-impermeable barrier in electrochemical cells,where in a conventional electrochemical cell an electrolyte-impermeablebarrier would be needed to ensure products from the anode and cathodeare kept separate while allowing ion transport through theelectrolyte-impermeable barrier for example as is the case in commercialwater electrolysers.

BACKGROUND

In many electrochemical processes different products are generated atthe anode and the cathode electrodes. Because the electrodes are mostadvantageously located in the closest possible proximity to each other,the products (e.g. gases, in the form of bubbles) may mix, contaminatingeach other. The product generated at one electrode may also be convertedback to its reactant or destroyed if it contacts the opposite electrode.To avoid this possibility, electrochemical cells of this type typicallyemploy an electrolyte-impermeable barrier. The electrolyte-impermeablebarrier is a physical barrier that lies between the electrodes, eitherpartially or fully. Being impermeable to liquid electrolyte, theelectrolyte-impermeable barrier stops or hinders the products of theanode from mixing with the products of the cathode immediately aftertheir formation (e.g. the mixing of dissimilar gas-supersaturatedelectrolyte solutions generated at the anode and the cathode). Theelectrolyte-impermeable barrier is, nevertheless, also designed so as toallow for electrical communication between the anode and cathode. Thisusually occurs in the form of an ion current between the electrodes.Thus, for example, the electrolyte-impermeable barrier may be apolymeric, ion-exchange membrane that allows ions to move from one sideof the electrolyte-impermeable barrier across theelectrolyte-impermeable barrier to the other side of theelectrolyte-impermeable barrier (thereby closing the electrical circuitbetween the anode and the cathode), but not liquid electrolyte norassociated reaction products of the ions. An electrolyte-impermeablebarrier of this type is sometimes referred to as a “diaphragm”.Alternatively, the electrolyte-impermeable barrier may be an impermeablesolid material which partially but not completely partitions the anodefrom the cathode, and around whose side's ions may migrate between theelectrodes to thereby close the electrical circuit. Anelectrolyte-impermeable barrier of this type is sometimes referred to asa “skirt”, a “partition wall”, or a “chamber divider”.

To illustrate the need for and role of an electrolyte-impermeablebarrier, one may consider the representative case of water electrolysis.In this process, water is electrochemically split into oxygen gas at theanode and hydrogen gas at the cathode as per the half-reactions below

At the anode: 2 H₂O →O₂ + 4H⁺ + 4 e⁻ E⁰ _(ox) = 1.23 V At the cathode: 4H⁺ + 4 e⁻→ 2 H₂ E⁰ _(red) = 0.00 V Overall reaction: 2 H₂O → O₂ + 2 H₂E⁰ _(cell) = 1.23 V

As can be seen, hydronium ions (H⁺; also called ‘protons’) are generatedat the anode and must migrate to the cathode in order to close theelectrical circuit. Thus, the electrolyte-impermeable barrier in a waterelectrolyser must allow H⁺ions to move from the anode to the cathode butstop the water electrolyte and associated gas bubbles from movingbetween the anode and cathode compartments.

In modern-day water electrolysers, the electrolyte-impermeable barrierused is most typically a diaphragm comprising sulfonatedtetrafluoroethylene based fluoropotymer-copolymer material, sold underthe trade name Nafion™, which is a “proton-exchange membrane” (or “PEM”). Protons (H⁺) are readily able to migrate across such a PEM andthereby move from one electrode to the other. Liquid water electrolyte,and associated gas bubbles/molecules are, however, blocked from passingthrough the PEM polymer. In alkaline electrolysers, asbestos wovencloths have traditionally been used as electrolyte-impermeablediaphragms in the past.

According to an authoritative scientific review of water electrolysisissued by the Danish government lab, Riso, entitled “Pre-Investigationof Water Electrolysis” (PSO-F&U; (2008), Pre-Investigation of Waterelectrolysis, NE1-DK-5057, p. 39-49), the electrolyte-impermeablediaphragm in such an electrolysis cell must fulfil multiple roles,including the following:

-   -   (1) The electrolyte-impermeable barrier must prevent mixing of        gas-filled electrolyte from the cathode with gas-filled        electrolyte from the anode. Gas evolution at an electrode in an        electrochemical cell typically generates a two-phase mixture of        liquid electrolyte with dispersed bubbles. Mixing of the anode        and cathode electrolyte will result in mixing of the gases,        precluding the attainment of high gas purities and electrical        efficiencies.    -   (2) The electrolyte-impermeable barrier must form an effective        diffusion barrier for the gas molecules formed at each of the        anode and cathode, so as to thereby avoid contamination of the        gases by molecular diffusion across the electrolyte-impermeable        barrier.    -   (3) In the case of an elastic electrolyte-impermeable barrier,        the electrolyte-impermeable barrier may also be useful in        preventing the formation of an electrically insulating gas        bubble curtain at the front side of the electrodes. This is        achieved by locating the electrodes physically close to the        electrolyte-impermeable barrier, such that the bubbles are        rapidly swept off the face of the electrode.    -   (4) Most importantly, in order to avoid an uncontrolled increase        in the electrical resistance of the electrolysis cell, it is        critical that the pores of the electrolyte-impermeable barrier        should not become clogged with gas bubbles. This may occur when        mechanical forces drive gas bubbles into the mouths of the        pores, or when a gas-supersaturated electrolyte solution        spontaneously forms new bubbles inside the pores. In such cases,        bubbles may only form in small cavities of radius r if a certain        degree of supersaturation is established according to the        equation:

${P - P_{sat}} \geq \frac{2\sigma}{r}$

-   -   At 30-60 bar, the supersaturation pressures of hydrogen and        oxygen are believed to be no more than a few bars. Thus, for        electrolyte surface tensions of ca. 200 dyn cm⁻¹, pore diameters        of 1-2 micrometers will reliably avoid gas clogging of the        electrolyte-impermeable barrier.    -   (5) The electrolyte-impermeable barrier must also provide a        sufficiently high hydrodynamic resistance of more than ca. 5 cm³        centipoises (cm² bar s)⁻¹ so as to avoid mixing of oxygen        saturated electrolyte from the anode with hydrogen saturated        electrolyte from the cathode due to occasional, operational        pressure differences between the cathodic and anodic        compartments.    -   (6) The electrolyte-impermeable barrier must display a low        electrical surface specific resistance when immersed in the        electrolyte, ideally not exceeding 0.2 cm² so as to avoid high        ohmic potential drops within the electrolyte-impermeable barrier        at current densities around 1 A cm⁻².

Thus, their is a conventional understanding that electrolyte-impermeablebarriers are required in electrochemical cells. A key challengeexperienced in the water electrolyser industry, by way of example, isthe high cost of the most widely used eleetrolyte-impermeable barriermaterial, Nation™, which may routinely retail for prices of US$500-$1500per square meter at the present time. The excessive cost of theelectrolyte-impermeable barrier is beaten, in many water electrolysersonly by the still higher cost of the precious metal catalysts that mustbe used; for example, platinum, which is used in electrolysers withacidic electrolytes, currently trades for around US$1,300 per ounce onworld markets. In water electrolysers employing basic electrolyte, theelectrolyte-impermeable barrier is often the highest cost component.

Complicating this challenge is the fact that alternativeelectrolyte-impermeable barrier materials, which may be less costly,generally display higher resistance to ion (H⁺) transport when used in acell. This means that such alternative electrolyte-impermeable barriermaterials increase the energy requirement to drive the electrochemicalprocess.

The key limitation at the present time in respect of water electrolyserelectrolyte-impermeable barriers, is that many commercial waterelectrolysers operate most efficiently at current densities of 1500-3000mA/cm² at voltages of <3 V. At the present time however, only expensiveelectrolyte-impermeable barrier materials like Nation™ membranes arecapable of facilitating such current densities at these voltages.

It is for these reasons that the US Department of Energy (DOE) have,over many years, instituted well-funded and wide-ranging programsseeking to identify suitable, low-cost, low-energy, alternativematerials for use as electrolyte-impermeable barriers in waterelectrolyser cells.

The DOE has also funded extensive programs aimed at reducing the highcost of the catalysts used in water electrolysers, most particularly theplatinum employed in acidic electrolysers and the iridium oxide used inalkaline electrolysers. These two components comprise, by far, the majorand overwhelming cost of water electrolyser stacks.

Very similar challenges exist in a wide range of other industrialelectrochemical processes, including, for example, the chlor-alkaliprocess for manufacturing chlorine, which is one of the most widely usedelectrochemical reactions in the world. The obvious way to reduce thecapital cost of the cells in such cases, is to use a simpler, lessexpensive electrolyte-impermeable barrier.

In summary, the challenge of finding cheaper and more energy efficientalternatives to the electrolyte-impermeable barriers used in currentelectrochemical cells remains a problem, for which a solution is stillneeded.

The reference in this specification to any prior publication (orinformation derived from it), or to any matter which is known, is not,and should not be taken as an acknowledgment or admission or any form ofsuggestion that the prior publication (or information derived from it)or known matter forms part of the common general knowledge in the fieldof endeavour to which this specification relates.

SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the Examples. ThisSummary is not intended to identify all of the key features or essentialfeatures of the claimed subject matter, nor is it intended to be used tolimit the scope of the claimed subject matter.

In one form, the present invention provides an electrochemical cellwithout an electrolyte-impermeable barrier positioned between theelectrodes (i.e. between the anode(s) and the cathode(s)) of theelectrochemical cell. This is contrasted to a conventionalelectrochemical cell, where an electrolyte-impermeable barrier isrequired to be present to ensure products from the anode(s) and thecathode(s) are kept separate while allowing ion transport through oraround the electrolyte-impermeable barrier.

In another form there is provided an electrochemical cell comprising aliquid electrolyte, a cathodic and at least one cathode product able tobe produced at the cathode, and an anode and at least one anode productable to be produced at the anode. The at least one anode product and theat least one cathode product are substantially separated, and the cellis without an electrolyte-impermeable barrier positioned between thecathode and the anode.

The inventors have discovered that re-configuration of the componentsand/or their operating conditions within electrochemical cells, likeexemplar water electrolyser cells, provides for the advantageouselimination of any need for an electrolyte-impermeable barrier betweenelectrodes. This may be achieved without incurring a substantial energyand/or a cost penalty. Indeed, an overall energy and/or cost benefit mayinstead be realised.

Contrary to current practice, the inventors have recognised that atlower current densities, and preferably with appropriate, improved orideal electrolytes, there may be a relatively small energy penaltyassociated with increasing the inter-electrode gap. That is, with use ofa strongly ion-conductive electrolyte, the anode and cathode may belocated relatively far apart from each other in a cell, without creatingan excessive ion-conduction resistance and thereby incurring a largeenergy penalty to operating the cell.

Moreover, at lower current densities, each of the anode and cathode willtypically generate a relatively small stream of products (e.g. gasbubbles) per unit area. In the specific case of product streamscomprising gas bubbles that rise to the surface of a liquid electrolyte,two such well separated and small product streams can, additionally, becollected in different parts of the cell, thereby avoiding mixing of thegases. Cells may be specifically designed to separately collect thesmall and distinct streams of gas bubbles.

Alternatively and optionally, two such well separated and small productstreams can be directed to different locations within a cell forcollection, by ensuring that electrolyte which flows or is pumpedthrough the cell, sweeps the products (e.g. gas bubble streams) awayfrom each other, or otherwise maintains a separation between the productgas bubble streams, and to different compartments within the cell wherethey are separately collected.

This need not involve an additional cost, since virtually all suchelectrochemical cells already require circulating pumps. The onlyadditional cost, in this non-limiting example, is then incurred indesigning the cell so as to ensure that the electrolyte is pumped orflows along pathways that sweep the product streams to differentlocations for collection. Collection may involve; (i) in cases where theproducts are gases: coalescence of the gas bubbles and drawing off ofthe gas through a suitable gas outlet or valve; or (ii) in cases wherethe products are in the liquid phase: physical removal of theelectrolyte stream containing the products through a suitable outlet orvalve, for isolation or use of the products elsewhere. Various othermeans of collection may also be used.

In effect, and referring to an example only, the inventors haveunexpectedly realised that in the case where:

-   -   i. a small product stream is generated at the anode and/or the        cathode, and    -   ii. at least one of the product streams involves the generation        of as bubbles, and    -   iii. the anode and cathode are well separated,        then the physical features noted above that are associated with        the optimum electrolyte-impermeable barrier (e.g. maximum pore        diameter, supersaturation pressure, hydrodynamic resistance and        surface specific electrical resistance) are such that the        electrolyte-impermeable barrier is not required or may be        replaced with a barrier or structure that is        electrolyte-permeable. That is, there is no substantive need for        an electrolyte-impermeable barrier between the electrodes at        all. Alternatively, an electrolyte-permeable separator, or        structure, that is wholly or substanially permeable (e.g.        porous) to the liquid electrolyte may be located between the        electrodes in place of an electrolyte-impermeable barrier. For        example, a porous plastic sheet, that allows free movement of        the liquid electrolyte through the porous plastic sheet,        provides an electrolyte-permeable separator and may be used        instead of an electrolyte-impermeable barrier in electrochemical        cells of the present invention.

The distinction between an electrolyte-impermeable barrier, that stillallows ion transport, and an electrolyte-permeable separator can be verysignificant when considered from the viewpoint of cost. There are a verywide variety of available electrolyte-permeable separators that areporous to liquids; many of these are commodity materials that arealready manufactured in high volume and at low cost. By contrast, thereis a much more limited number of electrolyte-impermeable barriermaterials available and only a relatively small fraction of those haveion-exchange or other properties that make them suitable as anelectrolyte-impermeable barrier in an electrochemical cell. Thus, theregenerally will be a substantial cost advantage to using anelectrolyte-permeable separator in an electrochemical cell, if aseparator is used at all, rather than an electrolyte-impermeablebarrier. Still more inexpensive would be to not have anyelectrolyte-permeable separator between the electrodes.

Thus, in an example embodiment there is provided an electrochemical cellwithout an electrolyte-impermeable barrier and with anelectrolyte-permeable separator between the electrodes. In anotherexample embodiment there is provided an electrochemical cell without anelectrolyte-impermeable barrier and without an electrolyte-permeableseparator between the electrodes. Preferably, the electrochemical cellis an electro-synthetic cell (i.e. a commercial cell having industrialapplication) or an electro-energy cell (e.g, a fuel cell). In anotherexample, the cell utilizes abiological manufactured components.

In noting the above, the inventors recognised that there is, of course,a larger trade-off in cost, in that a cell of the above alternativedesign needs electrodes with a substantially greater surface area thandoes a conventional cell, in order to generate the same overall quantityof products. For example, a cell based on the above alternative approachand operating at a low current density of 10 mA/cm² would, in oneexample, have to employ about 180-times more electrode surface area thana conventional cell operating at 1800 mA/cm², in order to generate thesame overall quantity of products (assuming no changes in microscopicpore structure of the electrode material).

Furthermore, the inventors have recognized that another benefit ofoperating at low current density is that, at low current densities, onemay make use of inexpensive catalysts and electrodes, and stillfacilitate the electrochemical transformation with high energyefficiency. For example, in the case of water electrolysis, one mayavoid using very expensive precious metal catalysts, like platinum oriridium/ruthenium oxide, which are essential to achieving high energyefficiencies at high current densities. Instead, one may instead usecheaper, Earth-abundant materials, like nickel or manganese/cobaltoxides. At low current densities, the cheaper catalysts may readilyachieve or even surpass the energy efficiencies achieved by theexpensive catalysts at high current densities.

Alternatively, one may still make use of precious metal catalysts but atlow current density operation, one would typically require orders ofmagnitude less of the precious metal catalysts per unit area than isconventionally required. Low current density operation may, in this way,also result in lower overall costs.

Thus, example cells of the alternative designs and operation, may, infact, achieve better overall cost and energy efficiencies than existing,conventional electrochemical cell technology.

The inventors have further recognised that a cell having a largegeometric electrode surface area can only be operated viably, i.e.commercially, at a low current density if the ratio of the electrolytevolume (unit: m³) to the electrode surface area (unit: m²) of either thecathode or the anode, is relatively low. The electrode surface area, ofeither the cathode(s) or the anode(s) separately, refers to thegeometric surface area of the cathode(s) or the anode(s). The geometricsurface area is the macroscopic surface area of the cathode(s) or theanode(s) (i.e. not including microscopic pores that might provide ahigher electrochemically active surface area). For example, if the ratioof electrolyte volume to the geometric surface area of one of theelectrodes (electrolyte volume:electrode surface area) is 1:1, or infractional notation electrolyte volume/electrode surface area is 1 m(unit: metres), then a conventional cell operating at 1800 mA/cm² withan electrode, surface area of 1 m² and an electrolyte volume of 1 m³cannot be adapted to low current density operation at 10 mA/cm² toachieve the same overall output, since increasing the electrode surfacearea by 180-fold will require an increase in the electrolyte volume to180 m³, which would be impractical and unviable. For this reason, a celloperating at a low current density can only do so practically if it hasa relatively low ratio of electrolyte volume to electrode surface areaexpressed in fractional notation (unit: m), i.e. a relatively low ratioof electrolyte volume:electrode surface area. If there is an array ofcathodes, then the electrode surface area is the geometric surface areaof the cathodes in the array of cathodes. If there is an array ofanodes, then the electrode surface area is the geometric surface area ofthe anodes in the array of anodes.

In one example, the ratio of electrolyte volume to electrode surfacearea is less than or about 0.1 m (or 100 mm) (i.e. 1 m³:10 m²). Inanother example, the ratio is less than or about 0.01 m (or 10 mm). Inanother example, the ratio is less than or about 0.001 m (or 1 mm). Inanother example, the ratio is less than or about 0.0001 m (or 100 μm).In another example, the ratio is less than or about 0.00001 m (or 10μm). In another example, the ratio is less than or about 0.000001 m (or1 μm). In another example, the ratio is less than or about 0.0000001 m(or 0.1 μm). In another example, the ratio is less than or about0.00000001 m (or 0.01 μm). In another example, the ratio is less than orabout 0.000000001 m (or 0.001 μm).

In another example form, there is provided an electrochemical cell,comprising a cathode located in a cathode compartment and an anodelocated in a physically separated anode compartment, and at least twofluid passages allowing an electrolyte to flow between the cathodecompartment and the anode compartment.

In another example form, there is provided an electrochemical cell,comprising a cathode that in operation may produce a cathode product,and an anode that in operation may produce an anode product. The cellalso includes an electrolyte, and the cathode and the anode areseparated within the cell. Preferably, at least one product from thecathode, if any are produced, and/or at least one product from theanode, if any are produced, are directed to different locations.

In another example form, there is provided for the partial or completeelimination of a need for an electrolyte-impermeable barrier between theelectrodes in electrochemical cells, in which dissimilar products aregenerated at the anode and the cathode. In one form, this can beachieved by:

locating the anode(s) and the cathode(s) in substantially separatelocations within the cell, whereby:

a product stream from the anode(s), if present, and a product streamfrom the cathode(s), if present, are directed to different locations;and,

wherein the cell is operated at a relatively low current density.

Optionally, but not essentially, the cell may be so configured thatcirculating electrolyte separately sweeps the product stream(s) and/orintermediate ion(s) from the cathode(s) and/or the amode(s) to differentlocations within the cell, from where the products may be separatelycollected in pure or near-pure form.

Optionally, but not essentially, an electrolyte-permeable separatorthrough which liquid electrolyte is able to move freely, may bepositioned between or partially between the electrodes, such as beinglocated in the inter-electrode gap between the anode and cathode toassist with the complete separation of the product streams originatingfrom the anode and the cathode. The electrolyte-permeable separator isdistinguished from an electrolyte-impermeable barrier in that theelectrolyte-permeable separator permits free liquid electrolyte movementacross the thickness of the electrolyte-permeable separator, whereas aelectrolyte-impermeable barrier does not. An example of anelectrolyte-permeable, separator is a plastic sheet that is freelypermeable by a liquid electrolyte. Examples of such sheets include, forexample, woven polymer or natural fabrics having large, liquid-permeableholes/pores through the full thickness of the sheets.

Preferably, but not exclusively, the cell employs an electrolyte thathas a high ionic conductivity to thereby ensure a low overall resistanceto the electrical current.

Preferably, but not exclusively, the cell operates at a low currentdensity. This is preferably, but not exclusively, less than or about 10mA/cm². In an alternative embodiment, this is preferably, but notexclusively, less than or about 20 mA/cm². In an alternative embodiment,this is preferably, but not exclusively, less than or about 70 mA/cm².In a still further alternative embodiment, this is preferably, but notexclusively, less than or about 250 mA/cm². In additional embodiments,this is preferably, but not exclusively, less than or about 500 mA/cm²,or less than or about 1000 mA/cm².

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments will now be described solely by way ofnon-limiting examples and with reference to the accompanying figures.Various example embodiments will be apparent from the followingdescription, given by way of example only, of at least one preferred butnon-limiting embodiment, described in connection with the accompanyingfigures.

FIGS. 1(a) and 1(b) illustrate example tank electrochemical cells (e.g.electrolysers) without an electrolyte-impermeable barrier between theanode(s) and cathode(s).

FIG. 2 schematically illustrates an example electrochemical cell (e.g.electrolyser) without an electrolyte-impermeable barrier between theanode(s) and cathode(s) and having circulating electrolyte.

EXAMPLES

The following modes, features or aspects, given by way of example only,are described in order to provide a more precise understanding of thesubject matter of a preferred embodiment or embodiments.

In one example there is provided an electrochemical cell, comprising acathode that in operation produces a cathode product, and an anode thatin operation produces an anode product. The electrochemical cell,comprising the cathode, the anode and an electrolyte, is without anelectrolyte-impermeable barrier positioned between the cathode and theanode. The cell also includes an electrolyte, and the cathode and theanode are separated within the cell, and the cathode product and theanode product are directed to different locations. The cell can beoperated at a low current density, due to the ratio of a relativelysmall electrolyte volume to a relatively large electrode geometricsurface area. The ratio of electrolyte volume to electrode geometricsurface area can be expressed as electrolyte volume (m³):electrodesurface area (m²), or preferably the ratio can be expressed infractional notation as electrolyte volume (m³)/electrode surface area(m²). To be clear, reference to the electrode surface area refers toeither: the macroscopic geometric surface area of the cathode if thereis one cathode; the macroscopic geometric surface area of the cathodesif there is more than one cathode; the macroscopic geometric surfacearea of the anode if there is one anode; or the macroscopic geometricsurface area of the anodes if there is more than one anode. Hence, arelatively low ratio may apply to the cathode(s) and not the anode(s),to the anode(s) and not the cathode(s), or to both the cathode(s) andthe anode(s).

For example, the ratio is less than or about 0.1 m, less than or about0.01 m, less than or about 0.001, less than or about 0.0001, less thanor about 0.00001, less than or about 0.000001 m, less than or about0.0000001 m, less than or about 0.00000001 m, or less than or about0.000000001 m. The electrochemical cell is without anelectrolyte-impermeable barrier. That is, the electrochemical cell doesnot require or include a partial or full electrolyte-impermeable barrierbetween the cathode and the anode. In another form, the cell mayincorporate a partial or full electrolyte-permeable separator betweenthe anode and the cathode.

In a general example, there is provided an electrochemical cellcomprising a liquid electrolyte, a cathode and an anode. At the cathodeat least one cathode product is able to be produced. At the anode atleast one anode product is able to be produced. The at least one anodeproduct and the at least one cathode product are substantiallyseparated, or most preferably separated, after being produced. The cellis without an electrolyte-impermeable barrier positional between thecathode and the anode. By between, is also meant partially between, i.e.there is no electrolyte-impermeable barrier positioned between, whollyor in part, the cathode and the anode, or between part of the cathodeand anode or cathode(s) and anode(s) and anode(s) is electrode arraysare used).

In an example, the electrolyte flows past either the cathode or theanode, and/or the electrolyte exits the cell after flowing past thecathode or the anode. In another example the electrolyte circulatesbetween the cathode and the anode. The electrolyte can sweep an ionspecies away from the cathode or the anode. This means the cell does notrequire or include an electrolyte-impermeable barrier between thecathode and the anode.

In another example, the cathode and/or the anode have some degree ofporosity to enable electrolyte to pass through the cathode and/or theanode. For example, the cathode and/or the anode can be a series ofribbons of thin metallic foil, and the thin metallic foil can be of theorder of about 0.025 mm thick. A spacing between the ribbons can be inthe range of about 1 mm to about 20 mm. A spacing between the cathodeand the anode can be greater than 10 mm, greater than 35 mm, or greaterthan 90 mm. In other examples, the ribbons are coated withnano-particulates of a metal and a binder; the cathode and/or the anodeare made at least partly from nickel; the cathode and/or the anode aremade at least partly from titanium; or the cathode and/or the anode aremade all least partly from manganese or cobalt oxides. In furtherexamples, the cathode is located in a cathode compartment and the anodeis located in an anode compartment, and the cathode compartment and theanode compartment are physically separated.

Preferably, but not exclusively, the cell can be configured and operatedin a manner that maximises the energy and cost savings that can beachieved. Alternatively, the cell can preferably, but not exclusively,be configured and operated in a manner that achieves some energy andcost savings. Alternatively, the cell can preferably, but notexclusively, be configured and operated in a manner that is suitable inrespect of the energy and cost savings that can be achieved. Preferably,but not exclusively, the separation of the anode(s) and cathode(s) islimited to the minimum required for a reliable and complete separationof the products in more than 99.99% purity each.

In the representative case of a water electrolyser, where the productsare streams of hydrogen or oxygen bubbles, the anode(s) and cathode(s)can preferably, but not exclusively, be separated by more than 10 mm. Inan alternative embodiment, the anode(s) and cathode(s) can preferablybut not exclusively, be separated by more than 35 mm. In a still furtheralternative embodiment, the anode(s) and cathode(s) can preferably, butnot exclusively, be separated by more than 90 mm. Preferably, but notexclusively, the bubble streams from each of the anode and cathode canbe collected in separate compartments within the cell, within which thegas babbles can be allowed to coalesce to form a bulk gas phase thatwill then be collected, dried and stored.

Preferably, but not exclusively, low-cost. Earth-abundant catalysts andconductors can be used at the anode(s) and cathode(s). For example, inthe example case of a water electrolysis cell, cheap, Earth-abundantmaterials, like manganese or cobalt oxides can be used for the anodecatalyst and nickel used for the cathode catalyst. Preferably, but notexclusively, the cell can be fabricated out of low-cost materials. Forexample, in the case of a water electrolysis cell, the cell may befabricated out of low-cost polymeric materials which may be manufacturedusing low-cost manufacturing techniques, such as injection moulding orextrusion.

In one, non-limiting example embodiment, the cell is a waterelectrolyser of a tank design, containing near its base, at least onewater inlet regulated by a suitable valve, and containing near its top,at least two gas outlets regulated by suitable gas valves. In thisexample embodiment, the tank is separated at a defined height, forexample about two thirds of the way up, into two physically-distinctcompartments, each of which acts as the gas collection receptacle forgas bubbles from either the cathode or the anode electrodes of theelectrolyser, respectively. Each compartment is open at its base to theelectrolyte in the cell. Each compartment contains near its top, a gasoutlet regulated by a suitable gas valve. Directly below eachcompartment, are located closely-packed arrays ofelectrically-connected, conductive sheets or foils, coated withsuitable, inexpensive catalysts that serve as either the anodes or thecathodes of the cell (depending on which compartment they lie beneath).The purpose of having in each array, large numbers of very closelypacked conductive sheets or foils coated with suitable catalysts, is tomaximize the surface area of that particular electrode at the lowestpossible cost.

Preferably but not exclusively, the cathode and anode arrays are, attheir closest, physically separated by about 35 mm, or greater, thisbeing the minimum separation that ensures that bubble streams created ateach of the arrays only rise into the compartments for which they aredesignated.

Preferably, but not exclusively, the electrodes have a large surfacearea and, even more preferably, the electrodes have some degree ofporosity so that the electrolyte is made to pass through the electrodes(to thereby ensure near to complete reaction of a chemical species atthe electrode before the electrolyte is circulated back to the otherelectrode).

In particular examples, the cell has a relatively small volume ofelectrolyte, giving rise to a relatively low ratio of electrolyte volumeto electrode geometric surface area. For example, the ratio ofelectrolyte volume to electrode surface area, expressed in fractionalnotation as electrolyte volume (m³)/electrode surface area (m²), is lessthan or about 0.1 m (or 100 mm). In one example, the ratio is less thanor about 0.01 m (or 10 mm). In another example, the ratio is less thanor about 0.001 m (or 1 mm). In another example, the ratio is less thanor about 0.0001 m (or 1.00 μm). In another example, the ratio is lessthan or about 0.00001 m (or 10 μm). In another example, the ratio isless than or about 0.000001 m (or 1 μm). In another example, the ratiois less than or about 0.0000001 m (or 0.1 μm). In another example, theratio is less than or about 0.00000001 0m (or 0.01 μm). In anotherexample, the ratio is less than or about 0.000000001 m (or 0.001 μm)

In other examples, the ratio of electrolyte volume to electrode surfacearea, expressed in fractional notation as electrolyte volume(m³)/electrode surface area (m²), is in the range of, inclusively, fromabout 0.001 μm to about 0.1 m, or from about 0.001 μm to about 0.01 m,or from about 0.001 μm to about 1 mm, or from about 0.001 μm to about100 μm, or from about 0.001 μm to about 10 μm, or from about 0.001 μm toabout 1 μm, or from about 0.001 μm to about 0.1 μm, or from about 0.001μm to about 0.01 μm.

Preferably, there is no ion-conductive electrolyte-impermeable barrierbetween the electrodes/electrode arrays, either fully or in part.Optionally, but not essentially, there may be an electrolyte-permeableseparator located, either fully or in part, between theelectrodes/electrode arrays. An example of such an electrolyte-permeableseparator includes a polymer or natural fabric that allows freetransport of the electrolyte through the electrolyte-permeableseparator.

In one example, the electrode arrays are configured to not completelyfill the compartment above them, but to leave a headspace near the topof the chamber for the collection of gases. Preferably, the tank and thesupports for the conductive sheets or foils in each array, are made ofdurable, economic polymers. The polymers may, optionally, betransparent.

Optionally, there may be a water circulation system in the tank that isconfigured to separately sweep the bubble streams off each of thecathode and anode arrays in such a way as to ensure that the bubbles endup in their correct, designated compartment. In such a case, the minimumseparation between the anode and cathode arrays may be smaller than 35mm.

Optionally, the anode array and the cathode array may be in separatetanks, connected by suitable piping such that there may be additionalmeans to facilitate the removal of gaseous reaction product from theelectrolyte in one tank before the electrolyte is circulated back to thefollowing tank. Such additional means includes the use of reducedpressure, use of a media to facilitate the formation of gas bubbles, orthe use of a gas-liquid contactor.

Other configurations, involving other reactions, can be provided withoutan electrolyte-impermeable barrier. For example, flat-sheet, plate andframe configurations involving circulating electrolyte that separatelysweeps bubbles off multiple, distinct cathode or anode electrodes anddirects them into channels that are exclusively plumbed for hydrogen oroxygen bubble stream collection respectively.

In a particular example, the anode and/or cathode could be constructedaccording to the electrode examples discussed in International PatentPublication No. WO 2012/075546 for “Multi-Layer Water-SplittingDevices”, the disclosures of which are incorporated herein.

The following examples provide a more detailed discussion of particularembodiments. The examples are intended to be merely illustrative and notlimiting to the scope of the present invention.

Example 1 A Tank Electrolyser Without an Electrolyte-Impermeable BarrierWhich Separates Gaseous Products

FIG. 1(a) schematically depicts an example tank electrolyser 5 withoutan electrolyte-impermeable barrier. The electrolyser 5 includes apolymer tank containing a water inlet 10 near the base of electrolyser5, and two gas outlets—a first gas outlet 20 (for hydrogen collection)and a second gas outlet 3 (for oxygen collection) at the top of theelectrolyser 5. The gas outlets 20, 30 sit atop a cathode compartment200 and an anode compartment 300, respectively. For a top region, forexample the top one third of the tank electrolyser 5, the compartments200, 300 are separated by a solid polymer wall 40, as shown in FIG. 1.Note that, in this example, the wall 40 does not extend to any pointthat is directly between the anode and cathode.

Immediately below the cathode compartment 200 is located the cathodeelectrode array 50. Immediately below the anode compartment 300 islocated the anode electrode array 60. The arrays 50, 60 are placed sothat gas bubbles emanating from them rise into the respectivecompartment 200, 300 immediately above the electrode array 50, 60 andnot into the neighbouring compartment.

Each of the anode and cathode arrays 50, 60 include a series ofclosely-spaced ribbons of thin metallic foil. The foil is typicallytitanium or nickel, which is optimally around 0.025 mm thick. Theribbons are typically coated with nano-particulate metal, such asnickel, and a binder, for example a polymer such as afluoropolymer-copolymer (e.g. Nafion™) (typically 5% of the coating byweight). The ribbons are physically and electrically attached at one orboth ends to a metallic, 3D mounting bracket comprising numerous thinarms. The metal of the mounting bracket may be titanium, nickel, ormetal-coated stainless steel. Various types of mounting brackets may beused. For example, the mounting bracket may be a series ofclosely-spaced, parallel, thin, rails from which the ribbons are made tohang in haphazard arrangements (much like clothing on hangers may hangfrom the racks of a clothing store). The spacing between ribbons mayfall in the range of about 1 mm to about 20 mm. Ideally, but notnecessarily, the ribbons are able to move during the formation, releaseand buoyant rise of generated gas bubbles, to thereby ensure thatbubbles do not become blocked in the narrow spaces between the ribbons.

The minimum spacing between the cathode and anode arrays 50, 60 ispreferably about 35 mm at their nearest separation, this being as closeas they can be located in this configuration without bubbles of hydrogenending up in the anode compartment and bubbles of oxygen in the cathodecompartment.

Each mounting bracket and thereby also all of the ribbons which areattached in each electrode array 50, 60, are electrically connected toan external terminal. The cathode array 50 is attached to the externalelectrical terminal 500. The anode array 60 is attached to the externalelectrical terminal 600. The cathode and the anode each extend in asubstantially vertical direction. The cathode and the anode are alsosubstantially parallel to each other.

In order to operate the electrolyser 5, the tank is filled from waterinlet 10 with an electrolyte solution. The tank is filled up to thefill-line 70. The electrolyte solution can be 6 M KOH in the case of analkaline electrolyser, where the electrode arrays 50, 60 comprise ofnickel strips. Alternatively, the electrolyte solution may be a stronglyacidic electrolyte in the case of an acid electrolyser, where theelectrode arrays 50, 60 comprise of titanium or stainless steel strips.

A direct electrical current is now applied over the external terminals500 and 600. A voltage of 1.8 V would typically be applied such that alow current density of between, inclusively, from about 2 to about 20mA/cm² is achieved. The ratio of electrolyte volume to electrode surfacearea is, in this example, less than 0.027 m. The cell can also beoperated at other low current density values, for example less than orabout 1000 mA/cm², less than or about 500 mA/cm², less than or about 250mA/cm², less than or about 70 mA/cm², less than or about 20 mA/cm², orless than or about 10 mA/cm².

As a result of the applied electrical current, bubble streams ofhydrogen rise from the cathode array 50, into exclusively, the cathodecompartment 200. In the cathode compartment 200, the bubbles coalesceand pure hydrogen gas is collected at the gas outlet or valve 20. At thesame time, bubble streams of oxygen rise from the anode array 60 into,exclusively, the anode compartment 300. In the anode compartment 300,the bubbles coalesce and pure oxygen gas is collected at the gas outletor valve 30.

During operation, hydronium ions (H⁺; protons) freed by the oxidation ofwater molecules migrate from the anode array 60 to the cathode array 50.This migration is unimpeded in any way by the presence of any sort ofion-permeable and electrolyte-impermeable barrier (e.g. diaphragm)between the anode array 60 and the cathode array 50. Electrons releasedfrom the oxidation of water molecules on the catalytic surface of theanode array 60 travel through the external electrical circuit to thecathode array 50.

Thus there is provided an electrochemical cell 5, comprising a cathode50, an anode 60 and an electrolyte, without an electrolyte-impermeablebarrier positioned between the cathode 50 and the anode 60.

Hence, in this example embodiment, the tank electroyser operates asfollows.

-   -   Using the water valve(s), the tank is fed with water containing        suitable ion-conductive electrolyte up to the level of the        headspace in each chamber. A sensor may be used to detect and        maintain the water level in the electrolyser.    -   The water in the tank fills the spaces between the closely        packed conductive sheets or foils coated with catalysts that        make up each of the anode and/or cathode arrays.    -   A suitable current is passed through the anode and cathode        arrays, such that, while the overall current may be large, only        a relatively small current density is created at any one point        on the conductive sheets or foils coated with catalysts that        make up the arrays.    -   The dissimilar gases thereby generated at each of the arrays        (hydrogen at the cathode array and oxygen at the anode array),        form bubbles that rise in streams between the closely packed        sheets or foils coated with catalyst, to thereby fill the        headspace directly above the water in each compartment.    -   The gas collected in the anode compartment will then be pure        oxygen, while the gas collected in the cathode compartment will        then be pure hydrogen.    -   The accumulated gas bubbles in the headspaces atop each array        are separately allowed to coalesce and the pure gases are        collected by being drawn through the gas valves in each        compartment in the tank.

FIG. 1(b) illustrates an alternative embodiment of the electrolyser inFIG. 1(a). The only difference with FIG. 1(a) is that anelectrolyte-permeable separator 45 is present between the anode and thecathode in FIG. 1(b). The electrolyte-permeable separator 45 may be afine metal mesh (eg. a 150 LPI stainless steel mesh), a porous plasticsheet, or a fine polymer net or fabric (e.g. a polypropylene meshfabric). The electrolyte-permeable separator 45 is porous and readilyallows transport of the electrolyte through its thickness. In so doing,the electrolyte-permeable separator 45 does not impede electrolytemovement or block the movement of ions from the anode to the cathode, orvice versa. However, the presence of the electrolyte-permeable separator45 acts to diminish turbulence in the liquid electrolyte. In so doing,the electrolyte-permeable separator 45 helps ensure that the bubbles ofgas from each of the anode and cathode rise correctly into theirrespective collection areas or collection chambers. The tankelectrolyser 5, providing an electrochemical cell, can be used anelectro-synthetic cell (i.e. a commercial cell having industrialapplication) or an electro-energy cell (e.g. a fuel cell). The tankelectrolyser 5 utilizes abiological manufactured components.

Example 2 A Tank Electrolyser Without an Electrolyte-Impermeable Barrierin Which Circulating Electrolyte is Used to Separately Collect GaseousProducts

FIG. 2 depicts in a schematic form, another example tank electrolyser 15of similar design to the electrolyser 5 shown in FIG. 1, except that thecathode and anode compartments have been physically separated into twodistinct chambers—an anode chamber 310 (i.e. anode compartment) and acathode chamber 210 (i.e. cathode compartment). The cathode array 50 hasalso been moved to the front of its chamber, while the anode array 60has been moved to the rear of its chamber. The two chambers areconnected by passages, pipes, conduits or channels which allow theliquid electrolyte to circulate from one chamber to the other—via afirst or front pipe 80 and a second or rear pipe 90.

The operation of the electrolyser 15 shown in FIG. 2 differs from thatin Example 1 only in that the electrolyte is pumped between the twochambers 210, 310 in such a way that the electrolyte circulates from theanode chamber 310 along pipe 80 to the cathode chamber 210, and thenback again along pipe 90. In so doing, the electrolyte sweeps hydroniumions (H⁺; protons) that are generated at the anode array 60, to thecathode array 50, thereby facilitating and improving the necessaryion-conduction between the electrodes 50, 60. Indeed, if the anode array60 and cathode array 50 were located the same distance apart in a singletank filled with electrolyte, then the rate of ion migration betweenthem would be slower than the case where the pump was running anddriving the electrolyte through pipes 80 and 90. That is, all else beingequal, pipes 80 and 90 represent the shortest pathway for ion migrationby the protons between the anode array 60 and cathode array 50 when thepump is running. One effect of the circulating electrolyte is, arguably,to facilitate and speed up ion transport between the electrodes.

That sweeping motion of the circulating electrolyte also acts tofacilitate bubble formation and dislodgement at each of the anode array60 and the cathode array 50. Because these arrays are relocated in theirrespective chambers toward the inlet for the circulating electrolyte andaway from the outlet for the circulating electrolyte, any bubbles sweptoff each array have no option but to rise into the collection areadirectly above their respective electrode array. That is, the action ofpumping the circulating electrolyte around the cell acts to direct orrelease the bubbles into their correct collection area, therebyfacilitating complete separation of the gases. This is done without needfor an electrolyte-impermeable barrier between the electrodes. Indeed,the shortest pathway for ion-conduction between the electrodes when thepump is running, along pipe 80 or 90, is entirely free of anyelectrolyte-impermeable barrier—that is, there is noelectrolyte-impermeable barrier present or required.

Thus there is provided an electrochemical cell 15 comprising a cathode50 located in a cathode compartment 210 and an anode 60 located in aphysically separated anode compartment 310, and at least two fluidpassages 80, 90 allowing an electrolyte to flow between the cathodecompartment 210 and the anode compartment 310.

The schematic illustration of the electrolyser 15 in FIG. 2 is intendedto illustrate how circulating electrolyte may be harnessed and directedwith the intention of eliminating the need for anelectrolyte-impermeable barrier in a device like an electrolyser. Assuch, the separation that may be needed between the anode chamber 310and the cathode chamber 210 is exaggerated and is not to scale. This hasbeen done purely for the purpose of illustration. In fact, with theassistance of carefully directed circulating electrolyte, it is possibleto locate the cathode and anode arrays in very close proximity to eachother.

In another example, the electrode arrays need not be located inphysically distinct chambers. The electrode arrays could be positionedapart in an integrated single compartment or chamber that allows liquidelectrolyte to flow or be pumped between or past the electrode arrays.For example the integrated single compartment or chamber could be torusor doughnut-shaped, and could have a variety of cross-sectionalgeometries such as circular, square or rectangular. The electrode arrayscan be located to be diametrically opposite, and their respective gascollection areas, sections or chambers can be located above theelectrode arrays. The electrolyte can be caused to flow in one directionaround the integrated single compartment or chamber. A variation incross-section may be provided at different locations about theintegrated single compartment or chamber, for example in regions betweenthe electrode arrays the cross-sectional area may be smaller.

In a still further example, electrolyte-permeable separators, such asfine metal meshes (e.g. a 150 LPI stainless steel mesh) or fine polymernets or fabrics (e.g. a polypropylene mesh fabric) may be placed at theentrance to pipe 90 (in the cathode chamber) and/or at the entrance topipe 80 (in the anode chamber). The electrolyte-permeable separatorsallow free movement of the circulating electrolyte through them, but actto facilitate the bubbles from each of the anode and cathode rising intheir correct respective chambers for collection.

Example 3 An Electrochemical Cell Without an Electrolyte-ImpermeableBarrier in Which a Continuous Flow of Electrolyte is Used to Separateand Collect Products in the Liquid Phase

In strongly alkaline (caustic) environments (e.g. 1 M NaOH), hydrogenperoxide may be manufactured electrochemically. The process uses agas-diffusion electrode as the cathode and a conventional solid-stateelectrode as the anode. Oxygen is typically fed into the gas-diffusioncathode, thereby inducing the following half reactions when a suitablevoltage and current are applied (with suitable peroxide-formingcatalysts):

Cathode: 2O₂ + 2H₂O + 4 e⁻→2HO₂ ⁻ + 2OH⁻ . . . (1) Anode: 4 OH⁻ → O₂ + 2H₂O + 4 e⁻ . . . (2) OVERALL: O₂ + 2 OH⁻→ 2 HO₂ ⁻ E^(o) _(cell)0.476 V .. . (3)

As can be seen, this overall reaction consumes base, OH,⁻, and oxygen,O₂, to produce the hydroperoxide ion, HO₂ ⁻, which is the natural formof hydrogen peroxide under basic conditions.

A critical feature of this electrochemical process is that thehydroperoxide ion thus formed, is not allowed to contact the anode. Ifit does contact the anode, then the anode half-reaction changes to thatshown below;

Cathode: O₂ + H₂O + 2 e⁻→HO₂ ⁻ + OH⁻ . . . (1) Anode: HO₂ ⁻ + OH⁻→ O₂(pure) + H₂O + 2 e⁻ . . . (4) OVERALL: O₂ (air) → O₂ (pure) . . . (5)

That is, a suitable mechanism is needed in such a cell to keep thehydroperoxide ion formed at the cathode away from the anode, whilststill allowing OH⁻ ions formed at the cathode to migrate to the anode,where they are consumed.

In other words, if the hydroperoxide ion generated at the cathodemigrates to the anode, the cell will effectively waste the appliedelectrical energy to simply convert oxygen pumped in at the cathode intooxygen generated at the anode (equation (5) above).

A solution to this problem is to separate the anodes and the cathodes asdiscrete arrays similar to those described in Example 1, with acontinuous stream of 1 M KOH electrolyte pumped over and/or through oneor both of the electrode arrays. As a result, the hydroperoxide ionsgenerated at the cathode are swept away with the electrolyte and do nothave the possibility of contacting the anode. The four equivalents ofhydroxide ion (OH⁻) consumed at the anode in equation (2)) are providedby the continuous stream of 1 M NaOH, while the two equivalents of OH⁻produced at the cathode (equation (1)) are swept away with thehydroperoxy ions to thereby replace two of the four equivalents consumedat the anode.

Thus, before being passed over the electrode arrays, the electrolytesolution contains only 1 M KOH. After having been swept over theelectrode arrays, the electrolyte solution now also contains hydrogenperoxide. The resulting solution may, typically be used directly in apulp and paper mill. That is, the electrolyte is not circulated.Instead, the electrolyte solution is manufactured as a 1 M NaOHsolution, which is then treated by being passed through anelectrochemical cell which imparts the electrolyte solution with highconcentrations of hydrogen peroxide. The resulting solution is useddirectly for pulp and paper treatment.

This example therefore describes a situation in which a product in theliquid phase in an electrochemical cell is swept away from one electrodeby a continuous stream of electrolyte to prevent the electrolyte fromreaching the other electrode. For example, this may be achieved byappropriately positioning an inlet area and an outlet area for theelectrolyte in the cell, such as near one of the electrodes.

The cathode array in this example is preferably a set of closely packed,high surface area gas-diffusion electrodes, while the anode arraypreferably comprises of conductive ribbons of the type described inExample 1. A cell voltage of 1.6 V is preferably applied, resulting in alow current density of from about 2 to about 10 mA/cm².

Throughout this specification and the claims which follow, unless thecontext requires otherwise, the word “comprise”, and variations such as“comprises” or “comprising”, will be understood to imply the inclusionof a stated integer or step or group of integers or steps but not theexclusion of any other integer or step or group of integers or steps.

Optional embodiments may also be said to broadly consist in the parts,elements and features referred to or indicated herein, individually orcollectively, in any or all combinations of two or more of the parts,elements or features, and wherein specific integers are mentioned hereinwhich have known equivalents in the art to which the invention relates,such known equivalents are deemed to be incorporated herein as ifindividually set forth.

Although a preferred embodiment has been described in detail, it shouldbe understood that many modifications, changes, substitutions oralterations will be apparent to those skilled in the art withoutdeparting from the scope of the present invention.

1. An electrochemical cell comprising,: a liquid electrolyte; a cathode,at least one cathode product able to be produced at the cathode; and ananode, at least one anode product able to be produced at the anode;wherein, the at least one anode product and the at least one cathodeproduct are substantially separated, and without anelectrolyte-impermeable barrier positioned between the cathode and theanode.
 2. The cell of claim 1, where the cell is an electro-synthetic oran electro-energy cell.
 3. The cell of claim 1 or 2, wherein the cellutilizes abiological manufactured components.
 4. The cell of any one ofclaims 1 to 3, wherein the ratio of electrolyte volume to electrodegeometric surface area of the cathode or the anode (electrolyte volume(m³)/electrode surface area (m²)) is in the range of, inclusively, fromabout 0.001 μm to about 0.1 m.
 5. The cell of any one of claims 1 to 3,wherein the ratio of electrolyte volume to electrode geometric surfacearea of the cathode or the anode (electrolyte volume (m³)/electrodesurface area (m²)) is less than or about 0.1 m (or 100 mm).
 6. The cellof any one of claims 1 to 3, wherein the ratio of electrolyte volume toelectrode geometric surface area of the cathode or the anode(electrolyte volume (m³)/electrode surface area (m²)) is selected fromthe group of: less than or about 0.01 m (or 10 mm): less than or about0.001 m (or 1 mm); less than or about 0.0001 m (or 100 μm); less than orabout 0.00001 m (or 10 μm); less than or about 0.000001 m (or 1 μm);less than or about 0.0000001 m (or 0.1 μm); less than or about0.00000001 m (or 0.01 μm); and less than or about 0.000000001 m (or0.001 μm).
 7. The cell of any one of claims 1 to 6, wherein in operationthe cell has a low current density of less than or about 1000 mA/cm². 8.The cell of any one of claims 1 to 6, wherein in operation the cell hasa low current density of less than or about 500 mA/cm².
 9. The cell ofany one of claims 1 to 6, wherein in operation the cell has a lowcurrent density of less than or about. 250 mA/cm².
 10. The cell of anyone of claims 1 to 6, wherein in operation the cell has a low currentdensity of less than or about 70 mA/cm².
 11. The cell of any one ofclaims 1 to 6, wherein in operation the cell has a low current densityof less than or about 20 mA/cm².
 12. The cell of any one of claims 1 to6, wherein in operation the cell has a low current density of less thanor about 10 mA/cm².
 13. The cell of any one of claims 1 to 6, wherein inoperation the cell has a low current density of between, inclusively,about 2 to about 20 mA/cm².
 14. The cell of any one of claims 1 to 13,wherein an electrolyte-permeable separator is positioned at leastpartially between the cathode and the anode.
 15. The cell of any one ofclaims 1 to 13, wherein an electrolyte-permeable separator is positionedon the shortest pathway for ion-conduction between the cathode and theanode.
 16. The cell of any one of claims 1 to 15, wherein theelectrolyte flows past the cathode or the anode.
 17. The cell of claim16, wherein the electrolyte exits the cell after flowing past thecathode or the anode.
 18. The cell of any one of claims 1 to 16, whereinthe electrolyte circulates between the cathode and the anode.
 19. Thecell of any one of claims 1 to 18, wherein the cathode and the anode areseparated within the cell, and wherein the cathode product produced atthe cathode and the anode product produced at the anode are directed todifferent collection areas.
 20. The cell of any one of claims 1 to 19,wherein the cathode and the anode each extend in a substantiallyvertical direction.
 21. The cell of any one of claims 1 to 20, whereinthe cathode and the anode are substantially parallel to each other. 22.The cell of any one of claims 1 to 21, wherein the cathode and/or theanode are an array of electrodes.
 23. The cell of any one of claims 1 to22, wherein the cathode and/or the anode have some degree of porosity toenable electrolyte to pass through the cathode and/or the anode.
 24. Thecell of any one of claims 1 to 23, wherein the cathode and/or the anodeare a series of ribbons of thin metallic foil.
 25. The cell of claim 24,wherein the thin metallic foil is of the order of about 0.025 mm thick.26. The cell of claim 24, wherein a spacing between the ribbons is inthe range of about 1 mm to about 20 mm.
 27. The cell of claim 24 or 26,wherein the ribbons are coated with nano-particulates of a metal and abinder.
 28. The cell of any one of claims 1 to 27, wherein a spacingbetween the cathode and the anode is greater than 10 mm.
 29. The cell ofany one of claims 1 to 27, wherein a spacing between the cathode and theanode is greater than 35 mm.
 30. The cell of any one of claims 1 to 27,wherein a spacing between the cathode and the anode is greater than 90mm.
 31. The cell of any one of claims 1 to 30, wherein the cathodeand/or the anode are made at least partly from nickel.
 32. The cell ofany one of claims 1 to 30, wherein the cathode and/or the anode are madeat least partly from titanium.
 33. The cell of any one of claims 1 to30, wherein the cathode and/or the anode are made at least partly frommanganese or cobalt modes.
 34. The cell of any one of claims 1 to 33,wherein the cathode is located in a cathode compartment and the anode islocated in an anode compartment.
 35. The cell of claim 34, wherein thecathode compartment and the anode compartment are physically separated.36. The cell of claim 34 or 35, wherein the cathode compartment has anassociated cathode product outlet or valve.
 37. The cell of any one ofclaims 34 to 36, wherein the anode compartment has an associated anodeproduct outlet or valve.
 38. The cell of any one of claims 1 to 37,wherein the cell is a water electrolyser and the cathode product ishydrogen gas and the anode product is oxygen gas.
 39. The cell of anyone of claims 1 to 37, wherein the cell is used to manufacture hydrogenperoxide and the electrolyte flows past the cathode and sweepshydroperoxide ions formed at the cathode so that the hydroperoxide ionsexit the cell.
 40. An electrochemical cell, comprising a cathode, ananode and an electrolyte, wherein the ratio of electrolyte volume toelectrode geometric surface area of the cathode or the anode(electrolyte volume (m³)/electrode surface area (m²)) is in the rangeof, inclusively, from about 0.001 μm to about 0.1 m.
 41. Anelectrochemical cell, comprising a cathode located in a cathodecompartment and an anode located in a physically separated anodecompartment, and at least two fluid passages allowing an electrolyte toflow between the cathode compartment and the anode compartment.